The present invention relates to a color Doppler-type ultrasonic diagnostic apparatus which displays in color a distribution image of movement information for fluid, such as a flow of blood flowing in an object to be examined, or for a cardiac muscle of a heart, and in particular, to an improvement for display on a monitor of the apparatus.
A color Doppler-type ultrasonic diagnostic apparatus functions by detecting frequency shifts, caused by the Doppler effect of ultrasonic pulses transmitted toward an object being examined by means of frequency analysis of received echo signals, and displaying a distribution image of a flow of blood based on the detected results.
The detectable frequency shift (hereinafter, referred to as a Doppler shift component) in this apparatus corresponds to a component of velocity in an ultrasonic beam direction. For instance, as shown in FIG. 1, an ultrasonic beam is transmitted from an ultrasonic probe PB to a flow of blood BD in motion at a velocity of V. If .theta. is provided as an angle between the movement direction of the blood flow and the ultrasonic beam direction, the Doppler shift component, that is, the detectable velocity Va can be expressed as follows. EQU Va=V. cos .theta.
As is apparent from this expression, when the blood flow is in motion along the axis of the ultrasonic beam (i.e. the angle .theta.=0), the maximum Doppler shift component is obtained, leading to highly accurate measurement of velocity of the blood flow. On the other hand, deviation of the blood flow direction from the ultrasonic beam direction will cause the Doppler shift component to reduce by a value of "V-va" resulting from the angle .theta., thereby deteriorating measurement accuracy.
In order to avoid the problem of the angle .theta. when obtaining an absolute velocity of a blood flow (in this invention, "an absolute velocity" means "a velocity of a blood flow in its flowing direction itself" having its direction and magnitude as a vector value), there are several automatic correction methods. One of the correction methods obtains Doppler signals in two directions set apart by a small angle of .DELTA..theta. for an observing point and calculates an absolute velocity V. In another method a plurality of groups of piezoelectric transducers formed on a single probe are driven, through electric scanning, to obtain a plurality of Doppler signals by transmitting/receiving a plurality of ultrasonic beams along different directions to the same observing point, and the plural Doppler signals are used for an absolute velocity V.
In conventional display for corrected vector information of an absolute velocity of a blood flow, there is a widely known display method; a blood flow coming near a probe is shown in red or the like and a blood flow going away from the probe is shown in blue or the like.
During the diagnosis of abnormalities of blood flows, information which indicates whether a blood flow is in a direction away from a central portion, such as a heart, toward a peripheral portion, or in the opposite direction, is a matter of great importance.
However, blood vessels, including superficial blood vessels which run parallel to the body surface provide approximately parallel flows to the body surface and are thus approximately perpendicular to the ultrasonic beam. Accordingly, in the conventional color display (i.e., a display which selects either one of red or the like or blue or the like), the direction of the probe (i.e. the ultrasonic beam direction) affects largely the detection results of the blood flow.
In particular, a slight change in the beam direction to the blood flow causes the blood flow direction to fluctuate sensitively between the two directions of approaching the probe and going away from it. As the beam direction is frequently changed by operator handling of adjustment of a scanning angle in a linear scanning, such sensitive fluctuation requires the operator to pay excessive attention to the beam direction, creating much operation work.
In sector and convex scannings, the changes in the beam direction cause the displayed color on the same image to be changed between red and blue for even the same blood vessel. This makes it difficult to clearly recognize the directions of blood vessels on the image displayed. Namely, it is not easy to understand at a glance whether the direction of a blood flow is in a direction flowing away from a central portion (e.g. heart) toward a peripheral portion (e.g. internal organs) or in an opposite direction. In this case, an operator is required to adjust the ultrasonic beam direction or estimate the blood flow direction of an image shown by a monitor by using the operator's experience. This becomes an excessive operation.
Of course, while ignoring such excessive operation work or trouble resulting from changes in the beam direction, it is possible to color-display only the absolute values of blood flow velocities, without displaying the directions of the flows. But this is insufficient information for diagnosis, because flow directions aid in determining normal or abnormal directions of flows.
There is a further problem in the above mentioned conventional apparatus. In an image of a distribution of blood flows overlapped on a tomographic image, it is easy to estimate the direction of a blood flow when the blood vessel runs through the tomographic image over a long distance. However, if a blood vessel is across the tomographic image (plane), it is very difficult to consider the crossing angle therebetween and measure the direction of a blood flow. In such a case, two cross sections are necessary to be examined.